The proposed study will investigate the neuropsychological status of children treated for acute lymphoblastic leukemia (ALL) in a longitudinal fashion by means of serial neuropsychological, audiological, and electroencephalographic examinations and by comparison with control groups consisting of siblings of leukemic children and comparably aged children with Wilms' tumor. All children admitted to St. Jude Children's Research Hospital for treatment of ALL between the ages of four and twelve (40-50 per year) will be serially evaluated for five years by means of the Reitan-Indiana Neuropsychological Test Battery for Children or the Halstead Neuropsychological Test Battery for Children as well as electroencephalograms and the Staggered Spondeic Word Test. Identical procedures will be administered to the control groups. If alterations in neuropsychological functioning do occur, they are expected to be variable across age groups, consistent within age groups. A thorough statistical analysis of the data will result in the detection of areas of neuropsychological dysfunction and determination of the etiology of any dysfunctions which occur. The determination of specific areas of neuropsychological dysfunction will facilitate the development of remediation strategies applicable to the classroom. Academic achievement will be monitored yearly to assess the effectiveness of any remediation initiated. By thoroughly examining a population at risk of CNS dysfunction during the high risk period, new knowledge can be obtained not only concerning the population of ALL children but also of the developing CNS in general.